1. Field of the Invention
This invention relates to the field of tuning electrical circuits and more particularly to the tuning of electrical circuits on an integrated circuit device which includes gallium arsenide microwave devices.
2. Description of the Prior Art
In designing wide band microwave monolithic integrated circuits (MMIC) to achieve a flat frequency response over a wide band (greater than 3:1 bandwidth), the most difficult task is to produce a device with a comparable flat frequency response to that of a hybrid microwave integrated circuit (MIC). In principle, an MMIC should have tighter control of the various parameters than an MIC and therefore should have a flatter and more reproduceable frequency response than an MIC. In practice, however, the lack of accurate design tools and lack of reproduceability of field effect transistor (FET) characteristics require a large number of iterations before a wide band MMIC with a flat response is produced. However, the availability of bond wire tuning and other schemes have made wide band MIC devices with very flat frequency response routinely achievable. To reduce the number of design cycles and to improve the reproduceability of amplifier characteristics for an MMIC device, an on-chip tuning technique is required to achieve wide band performance.
In the field of MMIC devices, several prior attempts at on-chip tuning have been utilized, however, none are totally satisfactory. The first technique which has been used, is a mechanical severing of circuit connections made in air-bridges. For example, such a technique is illustrated in an article titled "MMIC: On-Chip Tunability" published in Microwave Journal, in April 1987, pp. 135-139, by Ravender I Goyal and Sarjit S. Bharj. In this article, the authors describe a scheme for tuning which involves having several circuit connections to a device, such as a spiral inductor or a group of capacitors, utilizing an air-bridge which extends above the wafer surface. In tuning a device of this type, based on measured circuit performance, the extra air-bridge connections are disconnected by mechanically scratching open the corresponding air-bridges to eliminate certain portions of the device. The extra air-bridges complicate the layout of the circuit, and most of the tunable connections need not be implemented in air-bridges if circuit topography is the only consideration; however, they are implemented in air-bridges because the suspended structure of the air-bridge is more accessible to mechanical disconnection. These disconnections are performed by an operator peering at the wafer through a microscope and even though care may be used in severing the air-bridge connection, the severed bridge structure can become shorted to other circuit elements causing incorrect circuit operation.
Another technique which has been used to tune a circuit by severing electrical connections which are added for the purpose of eliminating certain portions of a circuit element is the vaporization technique utilizing a laser beam. In the vaporization technique, the connections are "cut" by utilizing the power of the laser beam to vaporize the metal atoms of metallization connections. This has the disadvantage of forming debris on the wafer surface since the vaporized material redeposits on the wafer surface. Also, tuning of a circuit may be accomplished by the laser vaporization of, for example, a portion of an open stub, to change the circuit characteristic of a tuning stub. Again, however, the vaporized material redeposits on the wafer surface as was the case with the "cutting" process. This debris may act as an electrical short and may also lead to undesirable parasitic elements in the MMIC.
A third technique utilized in tuning of electrical circuits in an MMIC device involves a technique called laser assisted chemical reaction. This technique is described in detail in an article entitled "Adjustable Tuning for Planar Millimeter-Wave Circuits", published in the International Journal of Infrared and Millimeter Waves, Vol. 7, No. 11, pp. 1729-1746, by Dylan F. Williams, S.E. Schwarz, J.H. Sedlacek and D.J. Ehrlich. In the article, the authors describe three types of tuning methods, all of which utilize tuning by a shorting strip placed across a coplanar wave guide. In each of the three techniques, varying the position of a shorting strip changes the electrical characteristics, permitting circuit tuning. The most practical of the three tuning schemes, from a production standpoint, utilizes a shorting strip which is laser-etched to remove metal from the shorting bar, which is molybdenum. The laser stimulates a local chemical reaction in chlorine which is performed in a vacuum to form a volatile compound. No debris is formed with this technique, however the disadvantage is the need for vacuum and the handling of the corrosive chlorine gas. Another disadvantage of this type of tuning is that it requires that the microwave circuit performance be monitored at the time of tuning which requires microwave feedthrough to the vacuum chamber. In addition, it is very costly, if not impossible, to automatically step in vacuum the microwave probe over an entire wafer.